Tuberculosis is a contagious disease caused by Mycobacterium tuberculosis (Mtb). The primary areas of infection are the lungs, and the lower respiratory region. The infection occurs by airborne transmission of droplet nuclei containing viable, virulent organisms produced by sputum-positive individuals. The bacilli are then deposited in the alveolar spaces of the lungs, where they are engulfed by alveolar macrophages. Once the tubercle bacilli enter the air sacs of the lungs, they multiply and spread throughout the body through the blood stream. A portion of the infectious inoculum resists intracellular destruction in the macrophage, persisting and eventually multiplying and killing the macrophage. The disease develops in individuals whose immune system is unable to successfully destroy the initial infection, and is characterized by symptoms such as chronic cough, low grade fever, night sweats and weight loss. A world wide epidemic of Tuberculosis currently exists. Incidence of the disease roughly correlates with poor economic conditions in particular geographic areas, with the highest incidence in developing countries. Improving social and sanitary conditions and ensuring adequate nutrition are necessary components to strengthen the body""s defenses against the tubercle bacillus. Tuberculosis is also found to be prevalent in people infected with HIV. A significant factor contributing to Tuberculosis resurgence is the emergence of multi-drug resistant organisms. Failure to complete the full course of drug therapy can lead to Mycobacterium tuberculosis (Mtb) organisms that are resistant to one or more anti-tuberculosis drugs, severely limiting effective treatment options. Mtb is a rod shape bacillus that favors high oxygen tissue tension and a neutral pH for growth. Mtb has also demonstrated slow and periodic growth in test studies as well as a susceptibility to acidic pH, inside phagolysosomes of macrophages. The organism exists in different metabolic states corresponding to different tissue environments. Actively growing extracellular organisms make up the largest population of mycobacteria. A second environment is inside the macrophages, at acid pH, where the organisms grow slowly. A third environment is in solid carouse areas, at neutral pH, where the organism also grows slowly. Another bacterial population, considered truly dormant, cannot be inhibited by any drug, and may therefore be susceptible only to the host""s own cellular defense mechanisms.
Pyrazinamide (PZA) is one of the few effective anti-tuberculosis drugs used in therapy. It is a white crystalline powder with a pKa of 0.5 and a molecular weight of 123.11 g/mole, and formula C6H7NO. First synthesized by Hall and Spoerri in 1940, Yeager and associates reported the antimicrobial activity of the compound in 1952.
While PZA is active against so-called susceptible strains of Mtb, it is not active against strains that have developed resistance to PZA. (A morpholine derivative of PZA, morphazinamide (MZA) has been used as an anti-tuberculosis drug, but is not reported to be effective against resistant strains.) Neither is it active against the closely related organism Mycobacterium avium. (Heifets et al, Am. Rev. Resp. Dis. 134: 1287-1288 (1986). M. avium has become a serious cause of disseminated infection among patients with AIDS. M. kansasii is another disease-causing mycobacterium that is resistant to PZA. PZA is also not effective against Mtb organisms which have been phagocytized. For example, PZA has been found to be either weakly or clearly bacteriostatic in macrophages, with a reduced efficacy after 4 to 7 days.
The antibacterial activity of PZA has been related to the formation of pyrazinoic acid (POA), which is formed by the action of the enzyme pyrazinamidase. One postulated mechanism for the resistance of Mtb to pyrazinamide is based on the ability of the organism to suppress formation of pyrazinamidase. Antimycobacterial activity of PZA is pH-dependent with greater efficacy at lower pH. This pH dependence may be related to POA formation.
Therefore there is a need for an effective anti mycobacterial drug having activity against PZA-resistant Mtb, against M. avium, and against phagocytized organisms and having activity over a broad range of pH values.
Surprisingly, it has been found that amides of pyrazine-2-carboxylic acid display good activity against M. avium, PZA-resistant M. tuberculosis, and phagocytized M. tuberculosis, and these compounds also show activity over a broad range of pH values.
In one aspect, the present invention relates to a method for treating for treating diseases involving pyrazinamide-resistant mycobacteria, including tuberculosis involving pyrazine-resistant or phagocytized Mycobacterium tuberculosis , and diseases caused by Mycobacterium avium and Mycobacterium kansasii. The method comprises administering to a mammal in need of treatment a therapeutically effective amount of a compound of formula 
wherein
R1 is hydrogen haloalkyl,or lower alkyl;
R2 and R3 are independently chosen from alkyl, substituted alkyl, cycloalkyl, aryl, substituted aryl, alkylaryl and substituted alkylaryl, or R2 and R3 taken together form a five- or six-membered heterocyclic or substituted heterocyclic ring with nitrogen;
X is hydrogen, halogen, lower alkyl or xe2x80x94NR4R5; and
R4 and R5 are independently lower alkyl or cycloalkyl, or R4and R5 taken together form a five- or six-membered heterocyclic or substituted heterocyclic ring with nitrogen;
or a pharmaceutically acceptable salt thereof. The activity of the compounds of formula I against mycobacteria is independent of pH.
In another aspect, the present invention relates to pharmaceutical compositions comprising a compound of formula 1, or a pharmaceutically acceptable salt thereof.